Modification of the electron energy distribution function during lithium experiments on the National Spherical Torus Experiment

Highlights: ► We examine Langmuir probe signals during liquid lithium experiments. ► Two interpretation methods are used, classical and non-local. ► We find increased electron temperature and decreased fueling efficiency. ► Electron temperature rise is correlated with a hot-electron population. ► Results are consistent with heuristic model of plasma incident on absorbing surface. - Abstract: The National Spherical Torus Experiment (NSTX) has recently studied the use of a liquid lithium divertor (LLD). Divertor Langmuir probes have also been installed for making measurements of the local plasma conditions. A non-local probe interpretation method is used to supplement the classical probe interpretation and obtain measurements of the electron energy distribution function (EEDF) which show the occurrence of a hot-electron component. Analysis is made of two discharges within a sequence that exhibited changes in plasma fueling efficiency. It is found that the local electron temperature increases and that this increase is most strongly correlated with the energy contained within the hot-electron population. Preliminary interpretative modeling indicates that kinetic effects are likely in the NSTX scrape-off layer (SOL) plasma. The decrease in plasma fueling efficiency, increase in local temperature, and increase in hot-electron fraction are all consistent with an absorbing surface intercepting the SOL plasma.